TFS/C#: Logging a custom warning during the build process - visual-studio

Question

I am hacking around a problem we've created for ourselves. What I would like to do is log a warning in our TFS builds for any code that is instantiating a specific class. I don't want a run time warning (I've got one in place already), I want a build time warning that ProjectX is using BadClass.cs. The idea being it will give us an additional place to see things that need to be fixed once our hack is no longer needed.
So something like this:
public class BadClass
{}
public class OkClass
{}
public class MyBadService
{
var a = new BadClass(); <-- Logs a warning to the build output
}
public class MyOkService
{
var a = new OkClass(); <-- Does not log a warning
}
Edit:
I do not like the idea of using Obsolete; its a misnomer. We've already got code with Obsolete attributes and this would get lost in the noise. I don't want a generic warning that I can't control the message for. I want bright neon signs with klaxons firing and a thousand exclamation points in the message. Basically everything I can do short of failing the build. I'm using the #warning precompiler directive right now and its mostly doing what I want but it requires a human to remember to add the warning. I'm looking for something more automagic. I've seen third party libraries do stuff like this so I know its possible.

Answer 1

Why not just use the Obsolete attribute? It can generate a build warning for you:
https://learn.microsoft.com/en-us/dotnet/api/system.obsoleteattribute?view=netframework-4.8
You can even make it emit an error too if you want.

Answer 2

The answer could be negative I think.
It seems that you use or call msbuild.exe to build your C# projects. But as far as I know, MSBuild in fact calls csc.exe to build C# projects in build time.
So actually what you want is logging a warning when the compiler compile the C# code if it recognize somewhere in your code uses the BadClass in build time.
If you have the source code of BadClass in the same solution, add a project reference format to the xx.csproj which contains BadClass, and set a #warning in the BadClass it may generate the warning in build time.
But I think the scenario you're in is something like: You developed one Assembly and distribute it to your user, so you want it generates a warning when the user calls one BadClass in your assembly and builds his own project to remind him of taking care when using this bad class. If so, this is impossible for msbuild AFAIK. If I misunderstand anything, feel free to know me know :)
Update:
As Daniel and Johnson said, ObsoleteAttribute is enough to do this. Though no valid way to generate warnings from msbuild aspect directly, but msbuild will call C# compiler during build process, so generates a compiler warning can output to build output window.